Display screen or portion thereof with a graphical user interface



The sole FIGURE is a front view of a display screen or portion thereof with a graphical user interface, showing our new design.

The dotted and evenly-dashed broken lines shown in the drawings of FIG. 6 illustrating portions of the graphical user interface comprising a herringbone patterned background and graphic flight information including an icon and text in a bottom banner do not constitute part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 